Skip to content

Unit testing with Hollow objects

Hollow provides a hollow-test jar that contains some testing utilities to facilitate unit testing with Hollow objects.


This class allows easy creation of HollowReadStateEngine objects for use in unit tests. This is useful when paired with a TestHollowConsumer (see below), by itself while using lower-level Hollow APIs, or as a mechanism to create Hollow objects for use in test cases.

HollowReadStateEngine engine = new HollowReadStateEngineBuilder()
    .add(new Movie("foo"))
    .add(new Actor("bar"))


A common pattern during unit or integration tests is to require an instance of a HollowConsumer that can be manipulated, similar to how test fixtures or mocks are manipulated. hollow-test provides a TestHollowConsumer class to facilitate this.

A simple example is shown below. See the unit tests in TestHollowConsumerTest for further examples.

HollowWriteStateEngine stateEngine = new HollowWriteStateEngineBuilder()
    .add(new MyDataModelType("somestuff", 2))
// we will add the snapshot with a version, and make the announcementWatcher see this version
long latestVersion = 1L;
TestHollowConsumer consumer = new TestHollowConsumer.Builder()
       .withAnnouncementWatcher(new TestAnnouncementWatcher().setLatestVersion(latestVersion))
       .withBlobRetriever(new TestBlobRetriever())
consumer.addSnapshot(latestVersion, stateEngine);


Hollow Test Data API eases the creation of dummy data for testing.

The Test API can be generated using the HollowTestDataAPIGenerator class by passing in a HollowDataset or using the data model like-

where dataset could be a snapshot published by a prior producer run if available, or it could be initialized from the data model classes by doing SimpleHollowDataset.fromClassDefinitions(Movie.class) where Movie.class is the top-level Class in the data model. In this example an instance of the generated class InputTestData can then be used to construct a state for the consumer with dummy data for the test for e.g.-
InputTestData input = new InputTestData();
        .countries() // is a set 
            .up() // to parent
        .tags() // is a map
            .entry(TYPE, "Movie")
            .entry(GENRE, "action");

And a HollowConsumer can be initialized with the data state like-

HollowConsumer consumer = input
    .withGeneratedAPIClass(ClientAPI.class) // the client API used to access data in a Hollow Consumer and not the test data generator API that was generated in this example